The present invention relates to a thin film electroluminescent (TFEL) display panel and more particularly, to a novel protective fill fluid for a thin-film electroluminescent display panel shielded by a pair of glass substrates with a protective material disposed therebetween.
For general background information on TFEL panels, see the "EL Glass Catalog and Design Handbook," Planar Systems, Inc Beaverton, Oreg. 97006, the contents of which, to the extent necessary, are hereby incorporated herein by reference.
A conventional TFEL display panel is illustrated in FIG. 1, wherein the panel comprises a first transparent glass substrate a plurality of transparent electrodes 2 made of In.sub.2 O.sub.3 or SnO.sub.2, and the like, a first dielectric layer 3, an electroluminescent (EL) thin film 4, a second dielectric layer 5, a plurality of counter-electrodes 6 made of for example Al, spacers 10, and a counter-substrate or cover plate 11, which may be made of glass. See, for example, U.S. Pat. No. 4,213,074 to Kawaguchi et al.
As illustrated, the transparent electrodes 2 are arranged on the glass substrate 1 in parallel with each other. The counter-electrodes 6 are arranged so that they cross at a right angle relative to the transparent electrode 2 in a plane view. The cross points between each of the transparent electrodes 2 and the counter-electrodes 6 create a picture element (pixels) i.e. the image forming portion of the TFEL panel. A power source (not shown) is applied to the transparent electrode 2 and the counter-electrode 6.
The first dielectric layer 3 may comprise Y.sub.2 O.sub.3, TiO.sub.2, Al.sub.2 O.sub.3, Si.sub.3 N.sub.4, SiO.sub.2, and the like, which may be deposited for example by a sputtering technique or by electron beam evaporation. The EL thin film 4 may be made for example, from a ZnS thin film doped with an impurity, for example manganese. The second dielectric layer 5 generally comprises a material similar to that of the first dielectric layer 3.
The TFEL panel is generally provided with a sealing structure for the EL composite member which comprises the first and second dielectric layers 3, 5 and the thin EL film 4. The cover plate 11, together with the transparent glass substrate 1, provide the basic structure for sealing the EL unit. The cover plate 11 need not be transparent because viewing may be conducted through the transparent glass substrate 1. One or more spacers 10 may be employed for positioning the cover plate 11. An adhesive 12 is coated for bonding the transparent glass substrate 1, the spacer 10, and the cover plate 11.
An adhesive 12 is generally employed, which may be an epoxy resin or the like. Lead terminals 15 of the transparent electrodes 2 and the counter-electrodes 6 may be formed on the transparent glass substrate 1 and extended toward the cavity. A control circuit (not shown) is coupled to the lead terminals 15 to apply the power to the EL unit.
A protective substance 13 may be added to the cavity defined by the two plates 1 and 11. A protective substance 13 functions to preserve the TFEL panel, especially the EL unit. The protective substance may be a gas or a liquid, but liquids are preferred. See, for example, U.S. Pat. No. 3,330,982 to Dickson, and U.S. Pat. No. 4,447,757 to Kawaguchi et al. Typical protective gases include inert gases such as nitrogen, argon, and the like. Typical protective liquids include silicon oils or greases.
A spacer 10 may be employed, and it may be formed from an insulating plastic sheet made of for example, a polyacetal resin or a polyimide resin, or a silicon rubber, or a glass plate. Finally, at least one fill hole 14 is generally provided, for the introduction of the protective substance 13.
If desired, a dye material or other color agent may be added to the protective substance in the TFEL panel to provide a background which can aid in the display characteristics of the panel.
TFEL panels of the type illustrated in FIG. 1 are very susceptible to moisture and therefore must be properly protected.
Prior to the present invention, silicone oils or grease were typically employed as the protective fill fluid for TFEL panels. Typically the method employed for filling TFEL panels with these substances involved the use of a cover plate having at least one hole. The cavity of the TFEL panel was filled by pumping out the air or gas in the panel and then allowing gravity and atmospheric pressure to move the protective liquid, e.g., silicone oil, into the panel. The hole was then sealed off using a glass disc adhesively bonded over the fill hole.
This method has several disadvantages; (1) the method is time consuming and bubbles are occasionally left in the oil which demands an additional pump-out step; (2) silicone oil (and grease) is extremely difficult to clean off the glass surface making it very difficult to bond to the glass reliably for hole closing; (3) silicone oils (and greases) absorb up to approximately 0.1% or 1000 ppm water from the atmosphere. This water must be baked out and vacuum pumped out of the oil prior to filling the panel. Water is extremely degrading to TFEL panel performance and, therefore, must be kept at very, very low levels.